1. Field of the Invention
The present invention relates to video monitors and, in particular, to circuits for generating an electron beam focus voltage.
2. Description of the Related Art
Cathode ray tubes (CRTs) typically utilize CRT focus means to focus the electron beam to minimize distortion and other artifacts.
Such focus means adjust the focus of the electron beam with an electrical field, in accordance with a control voltage applied to the focus means by focus circuitry. The electron beam is typically focused by the focus means before being deflected by the horizontal and vertical yoke deflection coils.
Distortion addressed by the focus means is caused, for example, by changes in the deflection angle of the electron beam of the CRT as horizontal scan lines are swept across the CRT face or screen. Thus, for example, the deflection angle in the four corners of a rectangular monitor are different from the deflection angles on horizontal scan lines displayed on the center of the monitor. Such changing deflection angles cause variations in the focus of the electron beam, in part because the CRT face is roughly planar and rectangular, rather than spherical.
Various forms of focusing circuits have been developed to help change the focus of the electron beam at different horizontal and vertical points during the displaying of a frame or field, to reduce the effects of these and other variations on the focus of the electron beam. Such circuits are typically optimized for a given horizontal scanning frequency. For example, in the NTSC system, one focusing circuit may be optimized for the "1H" scan frequency of approximately 15,734 Hz. Another circuit may be optimized for the "2H" scan frequency of approximately 31,468 Hz. However, it may be desirable to utilize variable-frequency monitors, in which the horizontal scanning frequency of the monitor is dynamically switched to one of a plurality of optional horizontal scanning frequencies. Since prior art focusing circuits are typically optimized for use at a given horizontal scanning frequency, switching to other horizontal scanning frequencies will result in a degradation of electron beam focus.
One possible approach to implementing a suitable focusing circuit for use with multiple horizontal scanning frequencies involves multiple capacitances that can be switched into and out of the focusing circuit as a function of the horizontal switching frequency. This approach is undesirable in practice because it adds complexity to the focusing circuit. Furthermore, capacitors in the focusing circuit typically may be charged to voltages that are equal to approximately 1,000 V. Switching voltages of this magnitude into and out of the focusing circuit is also undesirable.